The structural polysaccharides such as cellulose, lignin and hemicellulose are major components of plant cell walls. Over 2,000 genes have been estimated to be required for these polysaccharide biosynthesis, assembly and structural maintenance.
Glycosyltransferases (GTs) were regarded as an important family of proteins participating in the synthesis of polysaccharides by transferring sugar moieties from an activated nucleotide sugar to a specific acceptor molecule. Based on the amino acid sequence similarity, GT has been classified into 99 families designated as GT1 to GT99. These classifications are available in the carbohydrate active enzyme (CAZy) database (hhtp://www.cazy.org/GlycosylTransferases.html).
Pectin, hemicellulose and cellulose have been shown to be synthesized by at least one of GT members as summarized in three recent review articles. Galacturonosyltransferase (GAUT) 1, 7 and 8 that were classified as GT8 group members synthesize homogalacturonan of a pectin type among three different pectin polysaccharide types. Other pectin polysaccharide types of Rhamnoglacturonan I and II synthesized by arabinosyltransferase (ARAD) of GT47, galactosyltransferase (GAL) GT92, and xylosyltransferase of rhamnogalacturonan II (RGXT) of GT77 group. Xylogalacturonan synthesis, the third pectin polysaccharide type, was regulated by xylosyltransferase (XGD1) classified in GT47-C. Hemicellulose biosynthesis is regulated by CELLULOSE SYNTHASE-LIKE PROTEIN (CSL), IRREGULAR XYLEM (IRX) and CELL WALL MUTANT (MUR).
Most of these biosynthetic enzymes have been shown to localize to the Golgi apparatus, and all of them have predicted type II transmembrane topology. Although most GTs have specific activity synthesizing one of cell wall components, cellulose synthase (CESA) and CSL that were generally classified as GT2 and GT34, respectively, shared their activity both in cellulose and hemicellulose synthesis.
About 1.7% of Arabidopsis annotated genes were predicted as GTs but less than 20% of them were grouped into 42 GT groups.
An Arabidopsis GT14 member (AtGlcAT14A) was recently shown to function as β-glucuronosyltransferase involved in type II arabinogalactan synthesis. Domain of Unknown Function 266 (DUF266)-containing proteins (DUF266 proteins) share amino acid similarity with GT14 proteins but Pfam database annotates DUF266 as a plant-specific domain and predicts them as ‘likely to be GT related’. In Arabidopsis, a total of 14 DUF266 proteins (AtDUF266) were distantly related to GT14 group family. Recently, a total of 22 AtDUF266 proteins were identified by phylogenetic analysis of full-length amino acid sequences. Twenty-seven Populus DUF266 proteins (based on P. trichocarpa annotation v2.0) were also classified as GT14-LIKE proteins in this phylogenetic analysis. Again, these DUF266/GT14-LIKE proteins formed a cluster that was phylogenetically distinct from the GT14 family members. Subsequently, AtDUF266 proteins were categorized as ‘not classified GT (GTnc)’, to better reflect the uncharacterized features of this protein subfamily. The only characterized DUF266 protein is rice BRITTLE CULM 10 (OsBC10) which has amino acid similarity with 2 β-1,6-N-acetylgalactosyltransferase (C2GnT) in animals.
In vitro enzymatic assay using Chinese hamster ovary cells revealed that OsBC10 has galactosyltransferase activity that is only ˜1% of animal C2GnT. Rice natural variants of OsBC10 displayed phenotypic abnormalities such as small size of plant body and tiller number and brittleness of plant body. Glucose content was decreased in Osbc10 mutant, and xylose, arabinose and lignin contents were increased, indicating that OsBC10 influences cell wall composition. OsBC10 was predicated to be a type II intercellular membrane binding protein and was shown to be localized in the Golgi complex.
Except OsBC10, no other DUF266 proteins have been functionally characterized. Thus, the function of a large number of GTs remains elusive. For worldwide requirement to replace fossil transportation fuel, plant biomass is spotlighted as an alternative energy source. Efficient sugars release such as glucose and xylose from plant biomass is a pivotal factor to produce abundant useful biofuel.